The present invention relates to a ceramic packaged semiconductor device with a semiconductor element contained in a hollow package of ceramic, and more particularly to a ceramic packaged semiconductor device with a ceramic substrate containing SiC as the main component and having a good adhesion to a conductor paste or a soldering glass.
Semiconductor devices with a semiconductor element such as IC, LSI, etc. contained in a package of ceramic material having an inside space and with lead conductors introduced into the package, the semiconductor element and lead conductors being connected with one another by bonding wires in the inside space of the package, are widely used, as are resin-sealed semiconductor devices. The problem of package type semiconductor device is a poor heat dissipation due to the use of the package. Obviously, the poor heat dissipation is a great obstacle to an attempt to make a semiconductor device with a larger capacity, a higher intergration density and a smaller size. Thus, a material with a low heat resistance is required for an insulating substrate of a semiconductor element in a ceramic package. Besides, a material for the insulating substrate must satisfy (1) a higher electrical insulation, (2) a substantially equal coefficient of thermal expansion to that of silicon, and (3) a higher mechanical strength. Sintered alumina is now used for the insulating substrate which can meet these requirements to some extent. From the standpoint of the thermal resistance, sintered alumina having a low thermal conductivity, such as 0.05 cal/sec.cm..degree.C., is not regarded as a preferable material for the insulating substrate for the semiconductor device, when attempting to make the integration density higher and the capacity larger.
As an approach for improving the heat dissipation characteristic of the ceramic packaged semiconductor device, a structure is proposed, in which a semiconductor element is mounted on the top of a stud of copper which extends through an insulating substrate to the exterior of a ceramic package, and a supporting plate of molybdenum is interposed between the semiconductor element and the copper stud to serve for mitigating thermal stress which may be produced due to difference in the coefficient of thermal expansion between the semiconductor element and the stud, and further a cooling fin is mounted on and around the copper stud. With this structure, all the heat conducting paths extending from the semiconductor element to the cooling fin are provided by metals having a high thermal conductivity, whereby a semiconductor device having an improved heat dissipation characteristic due to the reduced overall thermal resistance can be obtained. However, a semiconductor device suffers disadvantages in that an increased number of manufacturing steps are required due to a complicated structure requiring an increased number of components, the weight of the semiconductor device is increased due to the use of heavy components such as those of molybdenum and others, and a troublesome procedure is required for mounting a printed circuit or the like onto the device.
A SiC-based ceramic substrate having a high thermal conductivity and a high electric insulation as disclosed in Japanese Kokai (laid-open) Patent Application No. 66086/81, is light in weight and high in strength, as compared with the now widely used oxide ceramic substrate such as an alumina-based ceramic substrate, and the coefficient of thermal expansion of such SiC-based ceramic substrate is about one-half of that of the alumina substrate, and approximates that of silicon semiconductor elements. Particularly, the thermal conductivity is substantially equal to that of Al, and thus the SiC-based ceramic substrate has a good heat dissipation characteristic and is suitable as a substrate for semiconductor devices with silicon semiconductor elements.
To solder silicon semiconductor elements or form a conductor circuit on the SiC-based substrates for semiconductor devices, the SiC-based substrates are usually metallized with commercially available conductor pastes of a mixture of glass and metal such as Au, Ag, Cu, etc. In the case of substrates for packaged devices, bonding of a lead frame to a substrate with a solder glass, and sealing of an alumina ceramic cap to a substrate with a solder glass are used in addition to the metallizing.
However, the SiC-based ceramic substrate is generally poor in wettability with a molten metal or a molten glass, and thus has a problem that metallizing with a conductor paste with a high bonding strength to a substrate or bonding with a solder glass can be hardly obtained.
In the meantime, a thick film circuit is provided on an oxide-based ceramic substrate, such as alumina-based ceramic substrate, with a thick film paste by baking, for example, by applying a conductor paste to the substrate by printing and baking the paste, thereby forming at least one of the conductors or resistors from a mixture of a desired metal including a metal compound and glass, and further applying a glass paste onto the conductor or resistor by printing, and backing the paste, thereby overcoating the conductor or resistor. However, a thick film circuit having a high bonding strength has not been formed yet for a SiC-based ceramic substrate. For example, a high bonding strength cannot be obtained even by direct baking of the paste on a SiC-based ceramic substrate containing BeO as a sintering aid [Japanese Kokai (Laid-open) Patent Application No. 66086/81].